Did you finish reading the statement you responded to?
I say it's because it's too small of a footprint and is secure enough to keep their efforts at exploit limited to the dumb users.
When needing user interaction you are better off targeting the masses as opposed to a handful of users. Now if I know my target is using a Mac then I will tailor my attack to that system. Much like Charlie does. Who would have the resources to do such a thing? How about other nations? They can write viruses to attack Macs just as easy as Charlie does. And with over 20 exploits remaining on Charlie's list of known OS X vulnerabilities I would say it's not safe as you like to think it is.
Like what “populations” for example?